Although I myself do not have a pop addiction (coffee's more my problem), I lived with someone who did, and it was not any fun. If we went to visit anyone, that person would bring at minimum a litre of diet pepsi with them. If we drove anywhere, there had to be a bottle of it with us. That person didn't drink anything else- not water, not milk, not juice. Their teeth suffered enamel loss. Travelling was limited, because we had to not only bring pop with us for the journey (which impacted packing space), this person drank so much we had to always be close to another source of it. The more of it they drank, the more of it they needed to function. That person spent a ridiculous amount of money on feeding their addiction.
One day, that person woke up and decided they didn't want to live like that anymore, and quit it. Cold turkey. I was so impressed- having quit the coffee habit more than once, I could well understand the cravings, the headaches, the brain-fog, the crankiness, and the tiredness.
I'm not saying that everyone can quit cold turkey, and no one can force anyone to change an unhealthy behaviour. Everyone needs to come to that realization themselves.
Like any addiction, your brain needs to be re-wired to work without the substance. I believe it took about a week to 10 days for them to feel functional again. That person carried around a water bottle and filled it up constantly so that dehydration didn't add to the withdrawal symptoms.
One of the ways you can help end an unhealthy addiction- whether to sugar, aspartame, caffeine etc- is to go gradual. Much like weaning a breastfeeding baby, start with one change a day. Perhaps not your morning jolt of java, but maybe the one you have in the middle of the morning.
Have a glass of water or a hot cup of rooibos (full-bodied, but not caffeinated) tea instead, go for a quick walk around the block or up and down the stairs, and/or have a healthy snack such as a handful of almonds and a cheese string or a hardboiled egg and avocado sandwich.
Once that mid-morning fix is easy to miss, try another one, such as your mid-afternoon one, and follow the same procedures. It's not an easy process, and it won't happen overnight, but you will feel more empowered and energetic once you conquer it. Good luck! I'm rooting for you!